ABSTRACT Oral pre-exposure prophylaxis (PrEP) has transformed the field of HIV prevention, but rigorous adherence to daily drug regiments is required to achieve protective efficacy.1-3 Disproportionate efficacy of oral PrEP in women has also led to more stringent requirements for adherence in a population already disadvantaged by higher HIV incidence rates globally. Girls and young women contribute significantly to the face of HIV worldwide thus necessitating a suite of prevention strategies focused on females to make a meaningful impact on ending the HIV/AIDS pandemic. This includes increasing access and adherence to existing effective oral and topical PrEP strategies, as well as pursing alternative HIV prevention options. On-demand prevention strategies with an extended window of protective efficacy of at least 2-4 weeks that could be used intermittently would offer a unique prevention modality not addressed by current products. We have developed an innovative drug delivery platform for topical PrEP that uses electrospun fibers that show potential for sustained HIV prevention. In this project, we propose to investigate the origins and extend the duration of antiviral drug delivery from our drug- eluting fiber formulations into tissue. The scientific premise for our project scope is based on several key observations. First, we have completed end-user studies both within the U.S. and at international sites that have informed the size/shape, rheological and performance attributes of a fiber-based microbicide intended for on- demand HIV prevention. These studies and others indicate that it is feasible to design a fiber-based dosage form that reflects the needs of end-users, and that on-demand prevention products are highly desirable and preferred to daily oral or topical PrEP. Second, we have measured in pigtail macaques that a single vaginal dose of a triple ARV drug-eluting fiber maintains cervicovaginal tissue concentrations that would be predicative of biological efficacy for at least two weeks. We will investigate the origin of this sustained drug release, and expect to find that our fiber dosages recapitulate the particulate and amorphous solid dispersion properties of long-acting drug nanocrystals that are important for their dissolution and bioavailability. Finally, we have developed an integrase inhibitor prodrug that allows us to differentiate between formulated prodrug and released parent drug by LC- MS/MS. This prodrug is a promising candidate for milling into nanocrystals due to it low aqueous solubility (log P>3), and the parent drug has been previously shown to be effective for both pre- (PrEP) and post-exposure (PEP) prophylaxis when used topically. Based on these observations, our proposed scope of work will: Iterate the design of extended-release fibers formulating long-acting ARV drug nanocrystals (Aim 1), Optimize fiber formulations of solid drug nanocrystals by iterative evaluation of safety and tissue pharmacokinetics in nonhuman primates (Aim 2), and Validate protective efficacy from a repeated low-dose vaginal SHIV challenge (Aim 3). Our project framework leverages innovative attributes of our drug-eluting fibers to enhance their development for on- demand topical prevention with an extended duration of protection.